1. Field of the Invention
The present invention relates to dielectric filters, dielectric duplexers, and transceivers for use in micro-wave or millimeter-wave communications.
2. Description of the Related Art
With the shift toward using frequencies in the micro-wave band and the millimeter-wave band, a plane-circuit type dielectric filter including dielectric resonators has been proposed, in which installation of the dielectric resonators and formation of configurations thereof can be easily and elaborately conducted by patterning of an electrode.
FIG. 8 illustrates a first embodiment of a conventional dielectric filter. The figure shows an exploded perspective view of the conventional dielectric filter.
As shown in FIG. 8, a conventional dielectric filter 110a includes a dielectric substrate 120a having an electrode formed on mutually opposing surfaces thereof, a lower case 112, and an upper case 111. In the electrode formed on the front surface of the dielectric substrate 120a, electrodeless portions or openings 121a through 121e are formed, whereas at opposing positions in the electrode formed on the back surface of the substrate, other electrodeless portions (not shown) having the same configuration as those on the front side are formed. Dielectric resonators 122a through 122e are composed of the parts defined by the openings 121a through 121e and the upper and lower cases 111 and 112. The resonance frequency is determined by the configuration of the openings 121a through 121e and the thickness of the dielectric substrate 120a, and other well-known factors.
The lower case 112 is composed of a substrate 113 and a metal frame 114 mounted thereon, and the dielectric substrate 120a is mounted on the metal frame 114, inside of which a step 115 is formed. An electrode 116 is formed on a surface of the substrate 113. Furthermore, input-output micro-strip lines 130 and 131 are formed on the surface of the substrate 113 as input-output couplers, and an electrode (not shown) is formed on substantially the entire back surface of the substrate 113.
The dielectric substrate 120a is mounted on the step 115 inside the lower case 112, in which the substrate 120a is fixed by a conductive adhesive material or the like. The upper case 111 is fixed on the metal frame 114 of the lower case 112. When input signals are input to the micro-strip line 130, the micro-strip line 130 and the dielectric resonator 122a are electromagnetically coupled and the dielectric resonator 122a resonates in the TE010 mode. Since the adjacent dielectric resonators 122a through 122e are electromagnetically coupled as well, signals are output from the micro-strip line 131 on the output side. In this case, the dielectric filter 110a serves as a five-stage band pass filter.
The unloaded Q (hereinafter referred to as Q0) of the TE010-mode dielectric resonator is higher than the Q0 of a rectangular-slot mode dielectric resonator, which will be described below. For example, in the 26 GHz band, Q0 of a TE010-mode dielectric resonator is approximately 1900, whereas Q0 of a rectangular-slot mode dielectric resonator is approximately 700. As shown here, since Q0 of the dielectric resonator is higher when the TE010 mode is used, a dielectric filter with small insertion losses can be obtained.
A second conventional dielectric filter will be illustrated by referring to FIG. 9. FIG. 9 shows an exploded perspective view of a conventional dielectric filter, in which the same parts as those in the first conventional dielectric filter shown in FIG. 8 are given the same reference numerals and thus detailed explanations thereof are omitted.
In the conventional dielectric filter 110b shown in FIG. 9, the configurations of openings 123a through 123e of an electrode formed on a dielectric substrate 120b are rectangular, which are different from those in the first conventional example. These openings form dielectric resonators 124a through 124e. Making the configurations of the openings 123a through 123e rectangular permits the resonance mode to be the rectangular-slot mode. Since the rectangular-slot mode weakens the degree of confinement of the electromagnetic field, the coupling (hereinafter referred to as Qe) between the dielectric resonators and the input-output couplers, and the coupling between the dielectric resonators 124a through 124e can be facilitated.
Regarding the above description of FIGS. 8 and 9, an illustration will be given by referring to graphs shown in FIGS. 10 and 11.
FIG. 10 is a graph showing the relationship between Qe and the distances between the input-output couplers and the dielectric substrate, in which the solid line indicates the TE010-mode dielectric resonator and the broken line indicates the rectangular-slot mode dielectric resonator. In FIG. 10, as well as in FIG. 9, it can be seen that the rectangular-slot mode permits coupling between the input-output couplers and the dielectric resonators to be facilitated. FIG. 11 shows a graph indicating the relationship between the coupling coefficients and the distances between the openings of the electrode forming the dielectric resonators, in which the solid line indicates the TE010-mode resonator and the broken line indicates the rectangular-slot mode dielectric resonator. In FIG. 11, as well as in FIGS. 10 and 9, it is shown that the rectangular-slot mode permits coupling between the dielectric resonators to be facilitated.
Meanwhile, in the field of high-frequency technology, the demand for improved characteristics has recently increased, such that dielectric filters having insertion losses of approximately 2 dB or lower are now being required.
The invention provides an improvement in the insertion loss characteristic of a dielectric filter with respect to its specific band. "Specific band" is defined by the following formula: EQU Specific band=(design band width/design central frequency).times.100%
The response characteristics of a filter (insertion loss, and out-of-band attenuation) depend on design band width, the order of the filter, and the unloaded Q of a resonator forming the filter, etc. Relationships between the values of these parameters and the insertion loss and attenuation of the filter are as follows:
TABLE 1 Insertion Loss Design band width wide narrow Order of the filter small large Unloaded Q large small Insertion loss small large
TABLE 2 Attenuation Outside Passband Order of the Filter small large Attenuation outside small large the pass band
In designing response characteristics of a filter, the above relationships are considered and each parameter is adjusted.
As shown in the above Table 1, the wider the design band width, the smaller the insertion loss of the filter. That is, the insertion loss of a filter which exhibits a large specific band is small. Also, the smaller the order of the filter, the smaller the insertion loss; and the larger the unloaded Q, the smaller the insertion loss.
As shown in Table 2, the order of the filter affects the amount of attenuation outside the passband.
When using a TE010 mode resonator (circular shape) for forming a filter, a relatively small design band width can be realized. The unloaded Q of the resonator (1/the loss of the resonator) is large.
On the other hand, a rectangular slot resonator realizes a wide design band width. But, its unloaded Q is small.
An ideal resonator to minimize the insertion loss of a filter should be able to realize a wide design band width and a large unloaded Q as shown in Table 1. But, in practice, a TE010 mode resonator is not able to realize as wide a design band width as a rectangular resonator. This results in the filter having excessive insertion loss. Further, since the rectangular resonator has a wide band width, its unloaded Q is small, which also increases the insertion loss.
FIG. 12 is a graph showing the relationship between the specific band and the insertion loss in a conventional dielectric filter. In this figure, the solid line indicates the first conventional dielectric filter, and the broken line indicates the second conventional dielectric filter.
As shown in FIG. 12, in the first conventional dielectric filter using the TE010 mode, in which Q0 is higher and insertion losses are thereby reduced, since the coupling between the dielectric resonators and the input-output couplers and the coupling between the dielectric resonators are weak, the dielectric filter is usable in narrow specific band filters whose specific band is less than 1%.
However, the second conventional dielectric filter using the rectangular-slot mode can be used with a specific band in the range of 1% or greater. In the range of 2% or greater of the specific band, insertion losses are 2 dB or lower, so that the required characteristics are obtained in wide specific band filters whose specific band is 2% or more. However, insertion losses increase in the range of 1% to 2% of the specific band, so the rectangular resonator has not been used in filters having a narrow specific band, where the narrow specific band and the small unloaded Q result in a filter having a large insertion loss.